CAM Engineer’s Insights and Best Practices for HDI CPB

Due to the requirements of high-integration ICs and high-density interconnection assembly technology, HDI boards have pushed PCB manufacturing technology to new heights and become one of the hottest topics in PCB manufacturing technology! In various CAM productions of PCBs, those engaged in CAM production consistently find that HDI mobile boards have complex shapes, high wiring density, and high CAM production difficulty, making it difficult to complete quickly and accurately! Faced with customers’ requirements for high quality and fast delivery, through continuous practice and summarization, I have some insights into this, and I would like to share them with my CAM colleagues here.

Defining SMD is the first important point of CAM production

In the PCB manufacturing process, factors such as graphic transfer and etching will affect the final graphic. Therefore, in CAM production, according to the customer’s acceptance criteria, we need to compensate for lines and SMD separately. If we do not define SMD correctly, the finished product may have some SMDs that are too small. Customers often design 0.5mm CSPs in HDI mobile boards, with pad sizes of 0.3mm, and some CSP pads are blind holes, with the corresponding pads being exactly 0.3mm, making the CSP pads and the pads corresponding to the blind holes overlap or cross each other. In this case, careful operation is necessary to prevent errors. (Using Genesis2000 as an example)

Specific production steps:

1. Close the drill layers corresponding to blind holes and buried holes.
2. Define SMD.
3. Use the FeaturesFilterpopup and Referenceselectionpopup functions to find the pads including blind holes from the top layer and bottom layer, and move them to the t layer and b layer respectively.
4. In the t layer (the layer where the CSP pads are located), use the Referenceselectionpopup function to select the 0.3mm pads that touch the blind holes and delete them, and also delete the 0.3mm pads in the CSP area on the top layer. Then, according to the customer’s design of CSP pad size, position, and number, create a CSP yourself, define it as an SMD, and then copy the CSP pads to the TOP layer, adding the pads corresponding to the blind holes on the TOP layer. The b layer is made in a similar way.
5. Check the provided gerber files to find other SMDs that are missing or defined multiple times.

Compared with conventional production methods, this method has a clear purpose, fewer steps, and can avoid mistakes, making it quick and accurate!

Removing non-functional pads is also a special step in HDI mobile board

Taking an ordinary eight-layer HDI as an example, first, remove the non-functional pads corresponding to the vias on layers 2 to 7, and then remove the non-functional pads corresponding to the buried vias on layers 3 to 6.

The steps are as follows:

1. Use the NFPRemovel function to remove the non-metalized hole pads on the top and bottom layers.
2. Close all drill layers except the vias, set Removeundrilledpads in the NFPRemovel function to NO, and remove the non-functional pads on layers 2 to 7.
3. Close all drill layers except the buried vias on layers 3 to 6, set Removeundrilledpads in the NFPRemovel function to NO, and remove the non-functional pads on layers 3 to 6.

Using this method to remove non-functional pads is clear and easy to understand, making it most suitable for personnel who are new to CAM production.

About laser drilling

The blind holes in HDI mobile boards are generally micro-holes of about 0.1mm. Our company uses CO2 lasers. Organic materials can strongly absorb infrared light and burn holes through thermal effects. However, copper has a very low absorption rate of infrared light, and its melting point is high, so CO2 lasers cannot burn copper foil. Therefore, the “conformal mask” process is used to etch the copper foil at the position of the laser-drilled holes (CAM needs to make exposure hole film). At the same time, to ensure that there is copper foil at the bottom of the outer layer (laser-drilled holes), the distance between the blind holes and buried holes must be at least 4 mils. For this reason, we must use the Analysis/Fabrication/Board-Drill-Checks to find holes that do not meet the conditions.

Plugging holes and solder mask

In the layer stack of HDI, the outer layer generally uses RCC materials, which have thin dielectric thickness and low resin content. Experimental data shows that if the finished board thickness is greater than 0.8mm, the metallized slot is greater than or equal to 0.8mm x 2.0mm, or the metallized hole is greater than or equal to 1.2mm, one must make two sets of plug hole files. That is, plug holes twice, use resin scraping and plugging for the inner layer, and directly use solder mask ink for plugging before solder mask on the outer layer. In the solder mask production process, there are often vias falling on or near SMDs. Customers require all vias to be plugged, so during solder mask exposure, it is easy for the vias that are exposed or half-exposed to the pads to leak ink. CAM personnel must deal with this. Generally, we first move the vias away. If it is not possible to move the vias, follow these steps:

1. For vias where the solder mask is covered, add transparent dots on the solder mask layer that are 3 mils smaller than the finished hole on one side.
2. For vias that touch the solder mask opening, add transparent dots on the solder mask layer that are 3 mils larger than the finished hole on one side. (In this case, the customer allows a small amount of ink to cover the pad)

Shape production

HDI boards are generally delivered as panel boards, with complex shapes. Customers attach a CAD drawing of the panel arrangement. If we draw according to the customer’s drawing using Genesis2000, it is quite troublesome. We can directly click on the CAD format file .dwg and change the save type to “AutoCADR14/LT98/LT97DXF (.DXF)” in the “Save As” option, and then read the *.DXF file in the normal way as reading genber files. While reading the outline, we also read the size, position, and number of stamp holes, positioning holes, and optical positioning points, which is fast and accurate.

Milling the outline border

When milling the outline border, unless the customer requires exposed copper, in order to prevent the board edge from flipping copper foil, according to the production standard, it is required to remove a small amount of copper foil inward from the border. Therefore, there may inevitably be situations like that shown in Figure 2A! If the ends A do not belong to the same network, and the width of the copper foil is less than 3 mils (the shape may not be able to be made), it will cause an open circuit. This kind of problem cannot be seen in the analysis report of Genesis2000, so another approach must be taken. We can do a network comparison again, and in the second comparison, remove 3 mils more copper foil inward from the edge of the copper foil. If there is no open circuit in the comparison result, it means that the ends A belong to the same network or the width is greater than 3 mils (the shape can be made). If there is an open circuit, widen the copper foil.

Conclusion

The demand for high-integration ICs and high-density interconnection assembly technology has propelled HDI boards to the forefront of the PCB manufacturing industry. However, their complex designs and dense wiring present significant challenges during CAM production, often resulting in delays and inaccuracies. To ensure top-notch quality and timely delivery, CAM engineers have developed valuable insights through continuous practice and refinement.

Key areas of focus include accurate SMD definition, streamlined non-functional pad removal, precision laser drilling techniques, efficient handling of plug holes and solder masks, optimized shape production, and meticulous border milling. By systematically addressing these challenges and utilizing advanced CAM strategies, professionals can achieve greater efficiency and precision in HDI board production. The result is the delivery of superior-quality PCBs that meet the evolving demands of the market.